Dynamoelectric devices such as motors and generators are often classified according to the orientation of the lines of flux between the stationary and rotating elements. Thus, electrical generators and motors are referred to as radial or axial field (axial air gap) devices. Although radial field devices are in widespread use, axial field devices have not found wide acceptance.
Dynamoelectric machines of the axial air gap type generally have a plurality of wedge-shaped core segments arranged in a circular array. The core segments have their wider ends adjacent the outer periphery of the circular array, and their narrower ends adjacent the axial center of the array. Supports for the array may include a winding radially extending between the wedge-shaped core segments and enclosure adjacent the outer periphery of the array.
In recent years, some advantages of the axial air gaptype motor have been recognized. The machine length in the axial direction is shorter, and the rotor is of a disk-type, therefore the total weight of the motor is considerably less than that of radial air gap type machine.
Prior art designs for axial field (axial air gap) motors or the like as exemplified by U.S. Pat. No. 2,469,808 (Aske), were generally of a pancake design and offered low volumetric efficiency. Core losses which include eddy current and hysteresis losses were substantial and greatly contributed to the overall inefficiency of the motor design. The core of the motor is composed of a flat annular ring formed from a tight, spirally wound strip of electrical steel lamination. Axial field machines with flat disk design features offered low volumetric efficiency and therefore, high losses. Examples of axial field motors employing multiple rotor or stator designs were also set forth in U.S. Pat. Nos. 2,557,249 (Aske) and 2,550,571 (Littman).
U.S. Pat. No. 2,734,140 (Parker) discloses a motor utilizing spacing threaded rods for the stator core blocks. Such an arrangement is difficult to manufacture. U.S. Pat. No. 3,567,973 (Parker) employs central stators inside end rotors. This solution introduces a cooling problem due to the unchanged design of the stator assembly, specifically when imbedded inside a plastic medium.
U.S. Pat. No. 3,699,372 (Abe) includes rotor segments wedged into a disk face of a secondary conductor, specifically applied for a central rotor cage. A similar design feature is shown in U.S. Pat. No. 2,550,571 (Littman). U.S. Pat. No. 2,245,577 (Dickman) shows a central stator arrangement of a cage-type axial induction motor, where the stator segments are composed of soft iron cores and have stator windings on their outer ends projecting outward from the non-magnetic plate. U.S. Pat. Nos. 4,370,582 (Addicot) and 4,410,820 (Stanley) have repeatedly introduced the annular rotor core of the cage-type axial induction machine.
U.S. Pat. No. 3,275,863 (Fodor) employs a design for stator/rotor segments using powdered metal technology. U.S. Pat. No. 3,469,134 (Beyersdorf) introduces two structurally separate disk portions for either the stator or rotor construction of the axial air gap type machine with a central rotor arrangement. Such a machine has split core segments and multiple air gaps which reduce the core's active cross sectional area and decrease the output power of the machine. It is also too complicated to assemble.
The earliest central rotor configuration of the axial induction machine is found in U.S. Pat. No. 1,605,796 (Tanzler), in which a doubled rotor core inside end stators have a separate magnetic loop. U.S. Pat. No. 1,829,686 (Swendsen) employs either end stator or end rotor configuration. Pat. Nos. 2,573,283 (Seitz) and 2,824,275 (Kober) utilized two-unit feature.
U.S. Pat. No. 4,371,801 (Richter) deals with a multistator axial air gap alternator having output power or power factor control ability, by controlling the degree of phase misalignment of two stator assemblies. A detailed description of the axial air gap rotor/stator core elements and their geometric properties is found in U.S. Pat. No. 4,394,597 (Mas). U.S. Pat. No. 4,500,806 (Kanayama) reveals an arrangement of printed flat coils inside a thin and coreless central stator annular base of insulating material. U.S. Pat. No. 3,575,624 (Keogh) uses a wire wound disk armature wave form closed winding connected to commutator segments.
There is thus a need for an improved polyphase axial air gap type induction machine with speed control feature that exhibits increased output power, improved torque capability, reduced noise and vibration, and is simple to manufacture. The invention presented was developed to achieve these goals.